Abstract: A method and device are designed to deliver intensity-modulated ion beam therapy radiation doses closely conforming to tumors of arbitrary shape, via a series of two-dimensional (2-D) continuous raster scans of a pencil beam, wherein each scan takes no more than about 100 milliseconds to complete. The device includes a fast scanning nozzle for the exit of an ion beam delivery gantry. The fast scanning nozzle has a fast combined-function X-Y steering magnet, and is coupled to a rastering control system capable of adjusting the length of each scan line, continuously varying the beam intensity along each scan line, and executing multiple rescans of a tumor depth layer within a single patient breathing cycle. An in-beam absolute dose and dose profile monitoring system is capable of millimeter-scale position resolution and millisecond-scale feedback to the control system to ensure the safety and efficacy of the treatment implementation.